Excellent idea! I take it the ball would be in contact with the mirror and the mounting plate, keeping the up and down (normal) distance fixed while allowing small amounts of movement in the horizontal (tangent) plane. Like this:
--------------------------------------------~
|
| mirror
|
-------------------------------------------~
| / \ \
/ | ball | / <-- blob-o-silicon
\ \ / |
-------------------------------------------~
|
| mounting plate
|
--------------------------------------------~
(not to scale)
Chuck